1. Field of the Invention
The present invention relates to a method of manufacturing a thin film transistor and semiconductor devices such as a liquid crystal display, an image sensor and a memory including thin film transistors.
2. Description of the Prior Art
In a semiconductor device such as a liquid crystal display or an image sensor using thin film transistors, the control of the threshold voltage of the thin film transistor is important. Especially for an active-matrix type liquid crystal display which includes peripheral circuits, it is necessary to control the threshold voltages of p- and n-channel thin film transistors in the periphery circuits for decreasing the dissipation of electrical power and for driving at a faster rate. The threshold voltage is mainly controlled by the state at the interface between the semiconductor active layer and the gate insulator layer in the thin film transistor, and it is not easy to control the threshold voltage in a wide range.
Previously, channel doping techniques are used for controlling the threshold voltage of a thin film transistor. In the channel doping technique, the ion implantation process is used to introduce a very small amount of impurities into a semiconductor active layer. However, the size of the ion beam is small in the prior art technique. When impurities are introduced into a substrate having a large area as such used for a liquid crystal display or the like, the ion beam has to scan over the large substrate, and this limits the area to be processed and the throughput of the processing. Therefore, it is difficult to adopt the ion implantation technique for manufacturing thin film transistors on a large substrate. Furthermore if the ion implantation is carried out on an insulator substrate such as a silica or glass plate, a charge up phenomenon occurs or the substrate is charged due to the implanted ions, and this lowers the precision of the implantation. Therefore, it is desirable to introduce impurities while preventing the charge up of the substrate.